I am in the process of finishing an end table. It has been stained and had 1 coat of wipe on poly applied to it. I went to do a light sanding before applying the next coat of poly, I was using 220 sand paper and while I sanding it scratched the surface leaving some large marks. So I had to get out my sander and sand away the coat of poly and then restain it. I am assuming that I just applied too much pressure while sanding so that is why the surface became so marred.
My question is, will using 0000 steel wool instead of the 220 work? Or do I just need a lighter touch?
Like Dave said, sanding between coats is not a neccessity except to remove blemishes to the finish and in the case where you are using your finish to fill open grain. With poly, you will need to sand for adhesion purposes only if the previous coat has cured past a certain point. Wipe on poly is little more than the brushing poly with added thinner. What this means is that the stuff that is left on the wood after the carrier flashes off is about 1/2 as thick as the brushing stuff. Consider that when you do any sanding.
I would recommend using 400 or higher grit with a sanding block and also that a lubricant such as water with a couple drops of dish liquid or some boiled linseed oil. I would also recommend that you wipe on at least three coats before sanding and let that cure a couple days before sanding.
Just be sure the poly is 100% dry before you use steel wool. I got a little bit adhead of myself and steel wooled something before it got completely dry. I got little steel wool fibers stuck in the poly in spots. What a pain. I switched to high grade sandpaper, because I didn't want this to happen again.
Ditto on scotch brite pads. Steel wool is kind of a mess and can get stuck in wood pores and itself become something that makes the finish less than perfect.
220 is too coarse for sanding between coats, IMHO, I generally use 220 as the final sanding before finishing.
You can make your own wipe on poly! Just regular (oil based) polyurthane + naptha. Adjust naptha amount for the right consistency (maybe start with a small batch, 30% naptha, 70% poly and add one or the other until it gets where you like it).
There was a neat article, I think in a recent Fine Woodworking that talked about waxing technique over poly or laquer. It was a bit like how a french polish is done, you build up a pad, with the inside of the pad being paper towels that are moist but not dripping with water, and the outside being thin cotton or something, with wax on it. Something I'll have to try, supposed to get very smooth surfaces with it.
Definitely do not use 220 when sanding finish coats. Build at least two coats, as as stated, use a lubricant. i use water while sanding. I soak my 320+ grit paper in water as well as rinse it frequently.
You may have used too much pressure as well. You want just enough to be able to hold onto the paper.
If you did it dry, what likely happend that caused the larger scratches is that the poly literally melts under the heat of the friction of your sanding. As it cools a little bit, it becomes a large piece of abrasive that is much more coarse than the paper itself. Using a lubricant therefore keeps the friction down and reduces the likelihood of this happening.
FWIW, i start with 320 and go incrementally to 600 0r 800. This means about five or six coats with the wipe on poly. Turns out a real nice finish.
"Ever notice how good enough, is usually neither good nor enough?"
>If you did it dry, what likely happend that caused the
>larger scratches is that the poly literally melts under the
>heat of the friction of your sanding. As it cools a little
>bit, it becomes a large piece of abrasive that is much more
>coarse than the paper itself. Using a lubricant therefore
>keeps the friction down and reduces the likelihood of this
Yes Darrin that would be my guess as well... if you don't see white powder when you sand stop the surface will mar and bad words will be uttered.
As has been said before "wipe on" coats are thinner so I usually wipe on a coupla three coats and then rub over with a Scotchbrite gray.
like Rusty I prefer nylon scouring pads, Steel wool causes all sorts of problems that can be avoided using nylon.